To convert the reciprocating rectilinear motion of the piston rod into rotary motion of a crankshaft in a reciprocating piston machine such as a reciprocating compressor, it is known to use a mechanism known as a crosshead, to one side of which said piston rod is connected, to its opposite side there being pivoted by means of a gudgeon pin the connecting rod of the crank mechanism which operates said crankshaft, said crosshead being provided with shoes slidable along rectilinear guides.
It is apparent that said connection between the piston rod and the crosshead must be of sufficient strength to transmit the considerable alternating stresses due to the gas thrust and inertia of the moving masses without suffering damage. Again, the current tendency towards increasingly high operating speeds gives rise to the serious problem of increasingly reducing the weight of the crosshead in that, especially in the case of machines with a high rotational speed, an even minimal reduction in the reciprocating masses results in a considerable reduction of the interial forces which stress the machanism, thus considerably improving the machine usability. Moreover, for certain applications, such as compressors to be mounted on transportable package units which must be of minimum possible overall dimensions, arrangements must be used which limit the overall size of the crank mechanism and thus the crosshead to a minimum. In the current state of the art the piston rod is normally connected to the crosshead either directly or indirectly via a fixing flange, so that the forces are in all cases transmitted by the rod to the body of the crosshead and from here to the gudgeon pin.
The main drawback of this arrangement is that as the crosshead body has to transfer the entire high load transmitted by the piston rod to the gudgeon pin, it must be of adequate from and strength, which immediately places unsurpassable limits on size and weight reduction of the crosshead, resulting in a corresponding limitation in terms of the size of the overall machine, thus affecting proper usability of the compressor, which because of the high reciprocating masses and consequent considerable forces of inertia cannot be used at the high operating speeds currently requested.
The object of the present invention is to obviate said drawback by providing a crosshead with a special connection to the piston rod by which the size and weight limitations of known arrangements are overcome, to allow high operating speeds.
This object is substantially attained in that the fixing flange to which the piston rod is connected by a hydraulic clamping system. said flange being optimized to resist bending by virtue of its shape and material, is connected directly to the connecting rod gudgeon pin by clamping screws passing through corresponding holes in the crosshead body and in said flange and engaging in threaded bores provided in the gudgeon pin, said screws being tightened with a load much higher than the load which the connection has to transmit.
In this manner, because of the direct connection between the flange to which the rod is fixed and the gudgeon pin, not only is the piston rod load transmitted to the clamping screws and thus to the gudgeon pin itself practically without involving the crosshead body which can therefore be reduced in weight and size, but the crosshead body is now made to work exclusively under compression as it is compressed between the flange and gudgeon pin by the clamping screws under a load such as to ensure that a sufficient residual compression load is maintained on the crosshead body under all operating conditions. An important consequence of this is that the connection between the crosshead and gudgeon pin is no longer subjected to thrust reversal as in the known art, with a resultant considerable reduction in the alternating fatigue loads which, as in well known, result in undesirable stresses which can also cause fracture and therefore require the crosshead to be overdimensioned for safety reasons. Again, the fact that the crosshead body remains permanently compressed by the load between the gudgeon pin and flange provided by the screws means that under all operating conditions the gudgeon pin remains permanently pressed against the crosshead body always on the same side, ie the piston rod side. The rear part, on the crankshaft side, of the gudgeon pin holding eyelet in the crosshead is therefore not even slightly stressed by the gudgeon pin, with the result that the corresponding part of the crosshead can be drastically reduced with consequent considerable reduction in the weight and overall size of the crosshead, to result in a crosshead suitable for high operating speeds.